The present invention relates to power plants, and pertains particularly to a combined gas and steam turbine power plant having a system for maximizing the efficiency and utilization of low grade coal and the like.
There exists in the power generating industry a need for more efficient power plants for converting fossil fuels to electrical power with minimal pollution. The most efficient commercially available technology for power generation systems is the combined cycle gas steam turbine operating on premium fuels. However, the cost and uncertainty of the availability of premium fuels make premium fuel combined cycle power generating plants impractical. Therefore, there exists an even greater need for power plants capable of efficiently handling lower grades of coal that exists in abundance in many regions of the world.
Fossil fuels, particularly coal, are used to power the vast majority of the power plants throughout the world today. Vast reserves of coal exist which cannot be effectively utilized in this and other environmentally sensitive countries without great cost (e.g. the use of expensive equipment with scrubbers). A large percentage of these reserves are contaminated with sulphur and other impurities, which result in undesirable pollution of the environment when burned in a conventional manner. This requires the use of recently developed very expensive and somewhat inefficient equipment to utilize these coal deposits without undue pollution.
Many of these deposits of coal throughout the world are of a very low grade and virtually unusable with present energy conversion systems. These low grade fuels, because of their contamination with impurities, form such a high percentage of non-combustable ash and other pollutants that they are not useable in conventional combustors. While many of these fuels can be burned in fluidized bed combustors, the cost of these combustors are high, the operating temperatures are low, leading to low efficiency and high cost of construction and operation. Moreover, fluidized bed combustion systems are complex and expensive to build, maintain and operate.
Many of the impurities can be removed from the coal through various known coal benefication processes, so that the coal can then be burned in a conventional combustor with minimal scrubbing. These coal benefication processes are normally carried out in a processing plant at the mine, with the purified coal transported to a power plant and the reject left at the mine site. The processes normally begin with crushing and pulverizing the coal into particles of very small size, to free the constituents from one another, and enable them to be at least partially separated from one another. The component containing impurities can be removed from the essentially pure coal particles more economically with recently developed technology, and a substantially clean fuel produced that can be burned in conventional combustors. However, in many cases, this processing approach produces a high percentage of reject and is generally cost prohibitive.
Coal fired power plants are traditionally used to generate steam for powering steam turbines for generating electrical power. Circulating fluidized bed combustors burning low grade coal may be satisfactorily used for steam generation.
Attempts have been made in the past to power gas turbines from coal fuel. These attempts have involved gasification of a portion of the coal to produce a combustible gas for burners of a gas turbine. However, this requires that the combustible gas be clean in order to avoid deposits that clog nozzles and corrode or build deposits on the turbine blades. An example of a plant of this type is disclosed in EPO Publication No. 0 046 406 published 24 Feb., 1982.
Combined cycle power plants having both gas and steam turbines are known. These are primarily to provide flexibility to meet changing load demands and are traditionally fueled by multiple fuels. The steam turbines are typically fueled by coal or oil, and the gas turbines fueled by a premium fuel, such as natural gas or a clean fuel oil.
U.S. Pat. No. 4,686,832, granted to Miliaras on Aug. 18, 1987 proposes cleaning and separating coal at a power plant for powering steam turbines for generating electrical power. A clean component of the coal is fired in a conventional combustor for generating steam for powering the steam generators. The dirty component of the coal is fired in a fluidized bed combustor for preheating feed water. This approach reduces the recirculating fluidized bed combustion system investment normally required for burning low grade coals.
Accordingly, an improved power plant to overcome these problems of the prior art would be desirable.